1. Technical Field of the Invention
The present invention relates generally to an improved sealing of a ceramic heater designed to be built in a gas sensor which may work to measure the concentration of a given component of exhaust emissions from an automotive engine.
2. Background Art
FIG. 7 shows a typical example of a gas sensor designed to measure the concentration of one of exhaust emissions from automotive engines.
The gas sensor 60 has installed therein a ceramic heater 9 for heating a sensor element 65 up to a desired activation temperature. The ceramic heater 9, as illustrated in FIGS. 8 and 9, consists of a ceramic heater body 92, a pair of conductors 93 formed on and in the heater body 92 in a given pattern, and leads 941. The conductors 93 are equipped with terminals 931. The leads 941 are joined to the terminals 931 through brazing metals 91, respectively, for supplying electrical power to the conductors 93. For example, Japanese Patent Fist Publication No. 11-292649 (U.S. Pat. Nos. 6,118,110 and 6,121,590) discloses such a type of ceramic heater.
Referring back to FIG. 7, the gas sensor 60 also includes a hollow cylindrical housing 68 which retains the sensor element 65 therein. The ceramic heater 9 is disposed inside the sensor element 65. The sensor element 65 is exposed at an outer surface of a top end thereof to a gas chamber 610 into which the exhaust gasses are admitted and at an inner surface thereof to an air chamber 620 into which the atmospheric air is admitted. The ceramic heater 9 is exposed at the terminals 931 to the air chamber 620. A sealant 631 is disposed between the sensor element 65 and the housing 68 to ensure an air-tight seal therebetween in order to avoid leakage of the exhaust gasses into the air chamber 620.
However, in recent years, the temperature of exhaust gas of automotive engines has been increased in order to meet tightened legal requirements of emission control, thus resulting in increased thermal loads on the sealant 631 of the ceramic heater 9, which gives rise to a degrease in degree of air-tightness between the housing 65 and the sensor element 65. This causes the exhaust gasses to leak into the air chamber 620 so that corrosion-causing substances, such as nitrogen oxides, contained in the exhaust gasses reach the terminals 931 of the ceramic heater 9. Additionally, moisture contained in the exhaust gasses may be adhered to the ceramic heater 9 or condensed during stop of the engine, thereby resulting in corrosion of the joints 913 of the terminals 931 and the leads 941 and, in the worst case, disconnections therebetween.